For clarity: I mentioned the 386 addressing modes because fundamentally the ModRM encoding was designed to address the same code generation problem: efficiently encoding one instruction to compute base-plus-offset (-plus-immediate too) in addressing memory. This avoids having to compute an address first for what is one of the most common operations in application code. As it happened, Intel's trick was the better idea. Motorola's original register design was fine but not as good, the '020 madness didn't survive contact with the RISC pipeline.
With the exception of index scaling (which as already mentioned, was added in the 68020) ModR/M and SIB is a strict subset of the 68000 addressing modes. I don't see what this has to do with the address register/data register split though. The 386 only had 8 GPRs and only 7 of those could be used as a base or index register. The reason for the address/data split is to allow 16 registers without needing 4-bit register fields.
Apart from the overly complex memory indirect modes, I'm having a hard time seeing how the 386 ModR/M and SIB setup is superior to what was in the 68020. Twice as many registers (though with usage restrictions), PC-relative addressing and a cleaner encoding. Those first two things have been fixed in x86-64 (and with generally fewer usage restrictions), but at the cost of making the encoding even worse.
